The present invention relates generally to luminescent nanoparticles and methods for their preparation.
Semiconductor nanoparticles, such as CdSe crystals with diameters in the range of 1-7 nm, are important new materials that have a wide variety of applications, particularly in the biological arena. Of the many unique properties of these materials, the photophysical characteristics are some of the most useful. Specifically, these materials can display intense luminescent emission that is particle size-dependent and particle composition-dependent, can have an extremely narrow bandwidth, and can be environmentally insensitive; such emissions can be efficiently excited with electromagnetic radiation having a shorter wavelength than the highest energy emitter in the material. These properties allow for the use of semiconductor nanocrystals as ultra-sensitive luminescent reporters of biological states and processes in highly multiplexed systems.
Some bare nanocrystals, i.e., nanocrystal cores, do not display sufficiently intense or stable emission, however, for these applications. In fact, the environments required for many applications can actually lead to the complete destruction of these materials. A key innovation that increases the usefulness of the nanocrystals is the addition of an inorganic shell over the core. The shell is composed of a material appropriately chosen to be preferably electronically insulating (through augmented redox properties, for example), optically non-interfering, chemically stable, and lattice-matched to the underlying material. This last property is important, since epitaxial growth of the shell is often desirable. Furthermore, matching the lattices, i.e., minimizing the differences between the shell and core crystallographic lattices, minimizes the likelihood of local defects, the shell cracking or forming long-range defects.
Considerable resources have been devoted to optimizing nanoparticle core synthesis. Much of the effort has been focused on optimization of key physiochemical properties in the resultant materials. For example, intense, narrow emission bands resulting from photo-excitation are commonly desirable. Physical factors impacting the emission characteristics include the crystallinity of the material, core-shell interface defects, surface imperfections or xe2x80x9ctrapsxe2x80x9d that enhance nonradiative deactivation pathways (or inefficient radiative pathways), the gross morphologies of the particles, and the presence of impurities. The use of an inorganic shell has been an extremely important innovation in this area, as its use has resulted in dramatic improvements in the aforementioned properties and provides improved environmental insensitivity, chemical and photochemical stability, reduced self-quenching characteristics, and the like.
Shell overcoating methodologies have, to date, been relatively rudimentary. Shell composition, thickness, and quality (e.g., crystallinity, particle coverage) have been poorly controlled, and the mechanism(s) of their effects on particle luminescence poorly understood. The impact of overcoating on underlying luminescence energies has been controlled only sparsely through choice and degree of overcoating materials based on a small set of criteria.
Hines et al (1996) xe2x80x9cSynthesis and Characterization of Strongly Luminescing ZnS-Capped CdSe Nanocrystals,xe2x80x9d J. Phys. Chem. 100:468 describe the preparation of a ZnS-capped CdSe nanocrystal that exhibits a significant improvement in luminescence yields: up to 50% quantum yield at room temperature. Unfortunately, the quality of the emitted light remains unacceptable, due to the large size distribution (12-15% rms) of the core of the resulting capped nanocrystals. The large size distribution results in light emission over a wide spectral range. In addition, the reported preparation method does not allow control of the particle size obtained from the process and hence does not allow control of the color (i.e., emitted wavelength).
Danek et al. report the electronic and chemical passivation of CdSe nanocrystals with a ZnSe overlayer (Chem. Materials 8:173, 1996). Although it might be expected that such ZnSe-capped CdSe nanocrystals would exhibit as good or better quantum yield than the ZnS analogue, due to the improved unit cell matching with ZnSe, the resulting material remained only weakly luminescent (0.4% quantum yield).
Other references disclosing core-shell-type luminescent nanoparticles include Dabbousi et al. (1997) xe2x80x9c(CdSe)ZnS Core/shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites,xe2x80x9d J. Phys. Chem. B 101:9463, Peng et al. (1997) xe2x80x9cEpitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility,xe2x80x9d J. Am. Chem. Soc. 119:7019, and Peng et al. (1998) xe2x80x9cKinetics of II-VI and III-V Colloidal Semiconductor Nanocrystal Growth: Focusing of Size Distributions,xe2x80x9d J. Am. Chem. Soc. 120:5343. Issued U.S. Patents relating to core-shell nanoparticles include U.S. Pat. Nos. 6,207,229 and 6,322,901 to Bawendi et al. However, each of these references fails to provide any correction for structural mismatches in the lattice structures of the core and the shell.
Described herein is a method that provides, via the use of a reaction additive, a core-shell material displaying superior chemical, photochemical, and/or photophysical properties when compared to core-shell materials prepared by traditional methods. The method may produce shells that are better wed to the underlying cores. The method may also produce shells that are more electronically insulating to the core exciton. Additionally, this method may facilitate the controllable deposition of shell material onto the cores.
Accordingly, it is a primary object of the invention to address the above-described need in the art by providing a luminescent nanoparticle prepared according to a method comprising providing an isolated semiconductive core, admixing the core with first and second shell precursors, a solvent, and an additive. The additive may comprise a Group 2 element, a Group 12 element, a Group 13 element, a Group 14 clement, a Group 15 element, a Group 16 element or Fe, Nb, Cr, Mn, Co, Cu, and Ni. The reaction dispersion thus formed is heated to a temperature and for a period of time sufficient to induce formation of an inorganic shell on the semiconductive core.
It is yet another object of the invention to provide a method of preparing a luminescent nanoparticle. In the method, an isolated semiconductive core is provided and admixed with first and second shell precursors, a solvent, and an additive. The resulting reaction dispersion is heated to a temperature and for a period of time sufficient to induce formation of an inorganic shell on the semiconductive core.
It is still another object of the invention to provide a method of preparing a luminescent nanoparticle. In the method, first and second precursors are injected into a first solvent system that is maintained at a temperature sufficient to induce homogeneous nucleation. This nucleation results in the formation of a monodisperse population of individual semiconductive cores comprised of a first semiconductive material having a first lattice structure. Next, at least a portion of the monodisperse population of individual cores is used to form a core dispersion that also comprises a second solvent and potentially an additive precursor. The second solvent system may be the same as the first solvent system. First and second shell precursors (and potentially an additive precursor) are then added to the core solution, resulting in the formation of a shell on each of the individual cores, with an interfacial region located between the semiconductive core and the inorganic shell. The interfacial region is comprised of elements of the semiconductive core, the shell, and potentially an additive, as described above. The shell is comprised of a second material having a second lattice structure, and may optionally also comprise the additive.
It is another object of the invention to provide a luminescent nanoparticle comprised of a semiconductive core, an inorganic shell surrounding the semiconductive core, and an interfacial region therebetween. The semiconductive core is comprised of a first semiconductive material having a first lattice structure. The shell is comprised of a second inorganic material having a second lattice structure. The interfacial region can be comprised of components of the semiconductive core and the shell and an additional additive that might be capable of incorporation into both the first and second lattice structures, i.e., the core and the shell, respectively.
The core may be comprised of (a) a first element selected from Groups 2, 12, 13 or 14 of the Periodic Table of the Elements and a second element selected from Group 16 of the Periodic Table of the Elements, (b) a first element selected from Group 13 of the Periodic Table of the Elements and a second element selected from Group 15 of the Periodic Table of the Elements, or (c) a Group 14 clement. Examples of materials suitable for use in the semiconductive core include, but are not limited to, MgS, MgSe, MgTe, CaS, CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, Al2S3, Al2Se3, Al2Te3, Ga2S3, Ga2Se3, GaTe, In2S3, In2Se3, InTe, SnS, SnSe, SnTe, PbS, PbSe, PbTe, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, BP, Si, and Ge, and ternary and quaternary mixtures, compounds, and solid solutions thereof. Particularly preferred semiconductive core materials are CdSe, CdTe, CdS, ZnSe, InP, InAs, and PbSe, and mixtures and solid solutions thereof.
The inorganic shell may be comprised of (a) a first element selected from Groups 2, 12, 13 or 14 of the Periodic Table of the Elements and a second element selected from Group 16 of the Periodic Table of the Elements, (b) a first element selected from Group 13 of the Periodic Table of the Elements and a second element selected from Group 15 of the Periodic Table of the Elements, or (c) a Group 14 element. Suitable second materials include, but are not limited to, MgO, MgS, MgSe, MgTe, CaO, CaS, CaSe, CaTe, SrO, SrS, SrSe, SrTe, BaO, BaS, BaSe, BaTe, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, HgO, HgS, Al2O3, Al2S3, Al2Se3, Al2Te3, Ga2O3, Ga2S3, Ga2Se3, Ga2Te3, In2O3, In2S3, In2Se3, In2Te3, SiO2, GeO2, SnO, SnO2, SnS, SnSe, SnTe, PbO, PbO2, PbS, PbSe, PbTe, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, BP, and ternary and quaternary mixtures and solid solutions thereof. Preferred second materials are CdSe, CdS, ZnSe, ZnS, CdO, ZnO, SiO2, Al2O3, and ZnTe. Optionally, an organic or other overcoat that is selected to provide compatibility with a dispersion medium may surround the shell.
The additive is generally comprised of a material selected from the group consisting of Group 2 of the Periodic Table of the Elements, Group 12 of the Periodic Table of the Elements, Group 13 of the Periodic Table of the Elements, Group 14 of the Periodic Table of the Elements, Group 15 of the Periodic Table of the Elements, and Group 16 of the Periodic Table of the Elements, as well as Fe, Nb, Cr, Mn, Co, Cu, and Ni, and may also be found in the semiconductive core. The additive, which might be present in the interfacial region, may also be present throughout the shell. If present in the shell, the additive may be evenly distributed in the shell or may be present in a decreasing concentration in an outward direction from the semiconductive core. In some cases, the additive is selected to provide the interfacial region with a crystalline structure that serves as a transitional lattice structure between the lattice structure of the core material and the lattice structure of the shell material.
In one preferred embodiment, the semiconductive core is CdSe or CdTe, the inorganic shell is ZnS and the additive is Cd. In another preferred embodiment, the semiconductive core is CdSe or CdTe, the inorganic shell is CdS and the additive is Zn.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.